Many hollow plastic parts are currently manufactured by injection molding processes in which gas pressure is used to press a molding composition outwardly against the sides of a mold cavity to form the desired shape. A system for carrying out a process of this sort is described in U.S. Pat. No. 5,639,405 to Erikson. These processes may be used to manufacture parts having lower weights and reduced material costs as opposed to parts manufactured by injection molding techniques which do not use gas injection. The use of such processes also reduces mold shrikage, leading to parts having improved tolerances and better surface appearances.
One drawback to such processes is that the operating pressure in the mold cavity is sufficient to drive at least a portion of the molding composition into the pressurized gas supply during those periods when pressurized gas is not being supplied. As the molding composition accumulates and hardens in passages leading from the supply to the mold cavity, these passages may become obstructed. Typically, the accumulated material cannot simply be flushed from these passages. Instead, the injection molding apparatus must be disassembled to remove the obstruction.
Nelson U.S. Pat. No. 5,149,546 proposed a check valve which allowed fluid or gas to be injected into a molten plastic flow path while purportedly preventing or minimizing back flow of molten plastic into the fluid or gas supply system. The check valve comprised a valve body and a separate tapered valve member adapted to fit within a tapered valve chamber at the end of a bore through the valve body. It is believed that the handling of the loose, relatively small tapered valve member increased the difficulty of installing or replacing this check valve. Furthermore, the check valve shown in Nelson includes a significant amount of dead space located radially outwardly from the valve member. This space provides an undesirable site in which molding composition could accumulate and potentially interfere with the operation of the valve.
Therefore, there remains a need in the art for valves for an injection molding apparatus adapted to minimize the backflow of molding composition into the pressurized gas supply.